1. Field of the Invention
The present invention relates generally to systems and methods for drying and gasifying substances using the calorific value contained in the substances and it more specifically relates to systems and methods for processing wet, pasty, sticky substances, such as municipal wastewater treatment sewage sludge, into a workable, powdered product.
2. Description of the Related Art
The primary problem facing every municipal wastewater treatment facility is the cost-effective, energy-efficient and environmentally-sound disposal of sewage sludge, the end product of wastewater processing. North America produces about 0.21 pounds of sewage sludge (on a dry basis) daily for every man, woman and child. In the United States, there are approximately 16,000 municipal wastewater treatment plants, with more being build each year to accommodate the growing population base. The EPA estimates that these plants generate about 7-8 million dry tons of sewage sludge per year. Typically, the sludge is mechanically dried at the municipal wastewater treatment plant to a wet cake consisting of approximately 25% solids and 75% moisture before it is processed further or sent off-site for disposal or beneficial re-use. Thus the total volume of wet sewage sludge cake managed each year by municipal wastewater treatment plants amounts to about 28-32 million wet tons. Annual costs for processing this waste are in excess of five billion dollars.
Historically, municipal sewage sludge management disposal programs relied upon: land filling; surface disposal; incineration; ocean dumping (banned in the United States in 1992); and/or beneficial reuse through land application. Each of these methods may be relatively inexpensive, but each may have undesirable aspects which result in a negative long-term cost to the environment.
A major portion of the United States' sewage sludge has been sent to landfills. As a result, valuable landfill space for large communities has been either so reduced, or the odor and quality of the sewage sludge cake make it so undesirable, that a great proportion of the nation's sewage sludge is now hauled a considerable distance to its final disposition. For example, over 75% of New York City's sludge is sent out of state to places as far away as Colorado and Texas. Portland, Oreg. trucks approximately 75,000 tons per year over 200 miles to Hermiston, Oreg. to be land-applied for beneficial reuse.
Another substantial portion of the sewage sludge in the United States has been sent to surface disposal sites. Surface disposal sites include monofills; surface impoundments and lagoons; waste piles; dedicated disposal sites; and dedicated beneficial use sites. Surface disposal differs from land application in that the sewage sludge that is surface disposed is placed on the surface of the land, rather than applied to enrich nutrient-depleted or barren soil.
However, a large number of states now have site restrictions or management practices governing sewage sludge disposal. Federally, the Clean Air Act, which governs sewage sludge incineration and the disposal of its residual ash, has recently been amended (the “Clean Air Act Amendments”) to levy stricter air emission measures for incineration. Further, in 1993 the EPA published 40 CFR part 503 Sludge Regulations (503 Regulations) employing the EPA's “exceptional quality” sewage sludge program. The 503 Regulations established “Standards for the Use or Disposal of Sewage Sludge” applicable to all wastewater treatment facilities. The 503 Regulations establish requirements for the final use or disposal of sewage sludge when sewage sludge is: applied to land to condition the soil or fertilize crops or other vegetation grown in the soil; placed on a surface disposal site for final disposal; or fired in a sewage sludge incinerator. The 503 Regulations also direct that if sewage sludge is placed in a municipal solid waste landfill, the provisions of 40 CFR Part 258 must be met. These provisions cover, in great detail, all aspects of establishing, maintaining and monitoring such landfills. Almost all communities are pursuing alternatives to incineration and landfilling.
“Land application for beneficial use” is the application of sewage sludge to land, either to condition the soil, or to fertilize crops or other vegetation grown in the soil. Sewage sludge may be beneficially land-applied on agricultural land, forest land, reclamation sites, golf courses, public parks, roadsides, plant nurseries and home land and gardens. Under the 503 Regulations, sewage sludge products that meet stringent requirements, including sufficiently low concentrations of certain pathogens and pollutants, and minimal attractiveness to disease vectors such as insects and rodents, are considered by the EPA to be Class A, “Exceptional Quality” sewage sludge. Class A sewage sludge is treated by the EPA in the same manner as common fertilizers; thus, this material is exempt from federal restrictions on their agricultural use or land application. Sewage sludge falling short of the highest EPA standards may nevertheless qualify as Class B sewage sludge.
Sewage sludge that meet Class B requirements may also be applied to the land for beneficial use, but is subject to greater record keeping, reporting requirements and restrictions governing, including among other items, the type and location of application, and the volume of application. Sewage sludge applied to the land for agricultural use must meet Class B pathogen levels and, if applied in bulk, require an EPA permit. Although land-application for beneficial reuse has heretofore been the best alternative, even this has drawbacks, including substantial fuel and personnel costs, odor complaints from neighbors and wear-and-tear on roads. In addition, you cannot land apply sludge during the winter months in cold weather climates.
Under the 503 Regulations, sewage sludge disposed of by surface disposal is subject to increased regulation by requiring, among other things: restricted public access; run-off and leachate collection systems; methane monitoring systems; and monitoring of, and limits on, pollutant levels. Surface disposal differs from land application in that, sewage sludge placed in a surface disposal site is required to meet, at least, Class B requirements.
Over the past few years, there has been a movement toward surface disposal of wet sewage sludge. However, in large metropolitan areas and rural communities alike, proposals to land-apply wet sewage sludge have been met with great resistance from the public. Factors affecting the acceptance of land-application are local geography, climate, odors, contaminants, land use, transportation costs and regulatory constraints.
Thus, the current 503 Regulations-compliant alternatives in municipal sewage sludge disposal are: to destroy it through incineration; to land-apply it under heavy public scrutiny and an overbearing regulatory scheme; to convert it to a more desirable form through composting; or, to reduce the volume of sewage sludge using drying methods that have heretofore been exceedingly costly. Overall, drying would be most desirable, were it not for the cost in fuel and expensive equipment.
The challenge in drying a pasty, sticky, gelatinous and difficult-to-handle material like sewage sludge is in removing the moisture trapped inside. Typically, wet sewage sludge is processed to a 20-25% solid cake through mechanical dewatering methods such as using centrifuges and belt filter presses. In order to remove more of the moisture, you need to apply energy—generally in the form of heat—to the sludge.
The current state of thermal-drying technology in the wastewater treatment industry is dominated by two heat drying technologies: direct and indirect. Direct drying technology puts hot air in direct contact with the sewage sludge during the drying process. Indirect drying technology causes the sewage sludge to come into direct contact with a heated surface, as opposed to hot air. U.S. Pat. No. 6,256,902 describes a system for drying wastewater sewage sludge, and is incorporated in its entirety by reference.
A myriad of new treatment technologies for removing the moisture from sewage sludge are being developed for small-scale operation. Some of these employ ultrasonic, microwave, additional adapted plate and frame technology, and radiant heat processes. However, none of these new technologies, nor those described elsewhere, meet the high-volume sewage sludge-processing needs of the major wastewater treatment facilities throughout the world. Indeed, virtually every wastewater treatment facility is looking for an economical, energy-efficient and environmentally-sound technology which dries municipal sewage sludge and recycles its end product.